1. Field of the Invention
This invention relates to an improved process for removing hydrogen sulfide from gases. More particularly, it concerns an improved continuous process for converting hydrogen sulfide contained in gases to elemental sulfur and thereafter removing the elemental sulfur.
2. The Prior Art
The removal of hydrogen sulfide from liquid and gaseous streams, such as the waste gases liberated or generated in the course of various industrial chemical processes, for example, in the pulping of wood, and in petroleum refining, has become increasingly important in combating atmospheric pollution. Such waste gases not only have an offensive odor, but they may also cause damage and constitute a health hazard. It is now imperative in many localities to remove virtually all of the hydrogen sulfide from waste streams under the penalty of an absolute ban on continuing operation of the plant.
The use of chelated iron redox catalysts for the oxidative removal of hydrogen sulfide from gas streams and conversion of the hydrogen sulfide to elemental sulfur is well known in the art. In such processes, a gas stream containing hydrogen sulfide is contacted with an aqueous solution of chelated ferric ion. The solution absorbs the hydrogen sulfide and converts it essentially quantitively to elemental sulfur. The ferric ion which is reduced to ferrous ion by reaction with the hydrogen sulfide is then regenerated by contacting the solution with a gas containing elemental oxygen, such as air.
Representative disclosures of this process or variations upon it can be found in the following patents: U.S. Pat. No. 4,401,642, U.S. Pat. No. 4,393,037, U.S. Pat. No. 4,243,648, U.S. Pat. No. 4,036,942, U.S. Pat. No. 4,390,516, U.S. Pat. No. 4,315,403, U.S. Pat. No. 4,206,194, and U.S. Pat. No. 4,083,945. Other disclosures of interest can be found in U.S. Pat. No. 3,097,925 issued to Pitts Jr., et al, on July 16, 1963, U.S. Pat. No. 3,068,065 issued to Hartley, et al, on Dec. 11, 1962, U.S. Pat. No. 4,278,646, issued to Lynn, et al, on July 14, 1981, U.S. Pat. No. 4,036,942 issued to Sibeud, et al, on July 19, 1977, U.S. Pat. No. 3,933,993 issued to Salemme on Jan. 20, 1976 and U.S. Pat. No. 4,009,251, issued to Meuly on Feb. 22, 1977. In general, these processes employ two separate zones - a first absorber zone for converting the H.sub.2 S to solid sulfur by reaction with metal ion and a second oxidizer zone for regenerating the metal ion.
In these and other patents, the solid particulate sulfur is generally removed from the suspension by flotation or by a gravity-sedimentation mechanism such as filtration, centrifugation or the like. Alternatively, the sulfur is heated until it melts and is removed as a liquid. In the latter cases the separation relies upon sulfur's specific gravity of 2.00.+-.0.05 to carry the sulfur down and the aqueous liquid up. The gravity drawn sedimentation processes can have problems. The solid sulfur can cake and build up in the sedimentation zone. It can plug filter media and generally pose difficulties. Melting the sulfur can also have problems. The high temperature can degrade the chelate solution. Molten sulfur can freeze up and present special handling problems.
There is a need for an efficient process which can reduce H.sub.2 S to 1 ppm or below and permit the efficient recovery of solid sulfur.